In recent years, biofuels have caught some attention from the viewpoint of environmental issues such as global warming. As a result, flexible fuel vehicles (FFVs) that can run with a fuel including gasoline and bioethanol, for example, at any arbitrary blend ratio have already been put on the market. Ethanol contents of fuels for FFVs vary depending on the blend ratio of gasoline and ethanol for the fuels available on the market. Examples of such variations range from E25 (i.e., a blend of 25% ethanol and 75% gasoline) to E100 (i.e., 100% ethanol), or from E0 (i.e., 100% gasoline) to E85 (i.e., a blend of 85% ethanol and 15% gasoline). Note that E100 here includes E100 containing approximately 5% of water (i.e., 5% water and 95% ethanol) that has not been sufficiently removed through the distillation processes of ethanol which is still left there.
In such FFVs, the properties of their fuels vary depending on the ethanol content of the fuels. In other words, gasoline, which is a multicomponent fuel, has a boiling point falling within the range of 27° C. to 225° C. FIG. 2 shows a change in the distillation ratio of gasoline with temperature. As can be seen from FIG. 2, the vaporization ratio of gasoline is relatively high even if its temperature is relatively low. In contrast, ethanol, which is a single component fuel, has a standard boiling point of 78° C. Thus, ethanol at a relatively low temperature has a vaporization rate of 0%, which is lower than that of gasoline. On the other hand, ethanol at a relatively high temperature has a vaporization rate of 100%, which is higher than that of gasoline. Hence, when the engine temperature of an FFV is low, i.e., equal to or lower than a predetermined temperature, the fuel vaporizability in a cylinder decreases as the ethanol content of the fuel rises or the engine temperature falls. Specifically, if the vaporization rate is defined as the ratio by weight of the fuel contributing to combustion to the fuel fed into the cylinder, the vaporization rate decreases as the ethanol content rises or as the engine temperature falls. When the engine is run cold with E100, for example, there arises a problem that the low vaporization rate causes deterioration in ignitability and/or combustion stability of an air-fuel mixture. In particular, this problem is serious with the water-containing E100.
For example, PATENT DOCUMENT 1 discloses an FFV engine system which extracts a fuel having a high gasoline content from a main tank storing a fuel that contains gasoline and ethanol at any given blend ratio; transfers the extracted fuel to a sub-tank which provided separately from the main tank; and stores the fuel in the sub tank. The engine system disclosed in PATENT DOCUMENT 1 allows the sub-tank to store constantly a fuel with stabilized vaporizability. Hence, when the engine system disclosed in PATENT DOCUMENT 1 uses a fuel having a high ethanol content, the system blends, at an appropriate ratio, the fuel stored in the main tank with the fuel stored in the sub-tank and having a high gasoline content, under a running condition (e.g., when the engine system is run cold) causing a decrease in the ignitability and/or combustion stability of the air-fuel mixture. Thus, the engine system injects, into an intake port of the engine, a blended fuel having a higher gasoline content than the fuel stored in the main tank. Consequently, the engine system disclosed in PATENT DOCUMENT 1 uses the fuel having a high gasoline content and stored in the sub-tank to increase the vaporization rate of the fuel, under such a running condition as to cause a decrease in the vaporization rate. Thus, the engine system ensures the ignitability and/or the combustion stability of the air-fuel mixture when the engine system is run cold.
On the other hand, PATENT DOCUMENT 2 discloses an FFV engine system without such a sub-tank. Instead, the engine system includes a fuel injection valve configured to directly inject a fuel into a cylinder. The engine system disclosed in PATENT DOCUMENT 2 increases a fuel pressure and injects the high-pressure fuel into a cylinder during the compression stroke at a cold start of the engine when the temperature of the engine and the vaporizability of the fuel are low and when the fuel has a high ethanol content and the fuel injection amount is large, in view of the fact that a theoretical air fuel ratio of ethanol is smaller than that of gasoline and the fact that the fuel injection amount needs to be increased when using a fuel having a high ethanol content than when using a fuel having a high gasoline content. With the increased fuel pressure, the fuel is atomized and the vaporization of the fuel is promoted. In addition, the timing of fuel injection is delayed, thereby starting combustion of the fuel before the injected fuel is caught on portions such as the inner wall and the piston of the cylinder. In this manner, the engine system disclosed in PATENT DOCUMENT 2 modifies the fuel injection modes so as to facilitate cold starting of the engine.